Nitrogen is a fundamental element, essential for life across all ecosystems. It forms the building blocks of proteins and nucleic acids, often a limiting factor for growth, especially in marine environments where its availability regulates primary productivity. While nitrogen is abundant in its gaseous form in the atmosphere, it must be converted into more reactive forms to be usable by most living things. This article explores the diverse natural and human-influenced pathways through which nitrogen travels from air and land into the ocean.
Nitrogen from the Atmosphere
Nitrogen compounds enter the ocean from the atmosphere through atmospheric deposition, a process with both natural and human-influenced contributions. Naturally, lightning converts atmospheric nitrogen gas into forms that can fall to Earth in precipitation. Certain marine microorganisms, like cyanobacteria, also convert atmospheric nitrogen into usable forms within the ocean itself through nitrogen fixation, making it available for other organisms.
Human activities have greatly intensified atmospheric nitrogen, increasing its deposition into oceans. The combustion of fossil fuels from vehicles, power plants, and industrial sources releases nitrogen oxides (NOx) into the air. Agricultural practices also contribute significantly, with ammonia volatilizing from fertilizers and animal waste entering the atmosphere.
Once in the atmosphere, these nitrogen compounds return to the Earth’s surface through two primary mechanisms: wet deposition and dry deposition. Wet deposition occurs when nitrogen compounds dissolve in atmospheric water droplets and are carried down in rain, snow, or fog. Dry deposition involves nitrogen gases and particles settling directly onto ocean surfaces or land without precipitation. Globally, reactive nitrogen deposition from the atmosphere to the open ocean has more than doubled over the last century, becoming a primary way anthropogenic nitrogen reaches these waters.
Nitrogen from Land and Freshwater Sources
Rivers and streams serve as major conduits, transporting substantial quantities of nitrogen from terrestrial environments to the ocean. A significant portion originates from agricultural runoff, where synthetic fertilizers and animal manure are washed into waterways by rain. This runoff often contains nitrate, a mobile form of nitrogen that easily leaches into streams. Wastewater discharge from sewage treatment plants and septic systems also introduces nitrogen into rivers and ultimately the ocean.
Urban areas contribute to nitrogen loading through stormwater runoff. Rainwater flowing over impervious surfaces in cities picks up nitrogen from sources such as lawns, pet waste, and atmospheric deposition, carrying it into drainage systems that lead to coastal waters. Industrial facilities also release nitrogen-rich effluents into aquatic systems. These land-based sources elevate nitrogen in freshwater bodies, which then flows to the marine environment.
Beyond surface runoff, nitrogen also reaches coastal waters through groundwater discharge. This involves the slower, continuous seepage of nitrogen-rich groundwater directly into marine ecosystems. Groundwater can become contaminated with nitrogen from agricultural practices, septic systems, and other human activities as it percolates through the soil. The nitrogen transported through these land and freshwater pathways can take various forms, including dissolved inorganic nitrogen like nitrate and ammonium, as well as dissolved organic nitrogen.
Consequences of Excess Ocean Nitrogen
Excessive nitrogen input into the ocean leads to a process known as eutrophication, where an overabundance of nutrients, particularly nitrogen and phosphorus, triggers rapid proliferation of algae and aquatic plants. This increase in primary production disrupts the natural balance of marine ecosystems. Eutrophication often results in harmful algal blooms (HABs), dense growths of certain algae species that can produce toxins.
These toxins can be detrimental to marine life, causing fish kills and impacting shellfish, and they may also pose risks to human health through contaminated seafood or direct exposure. Beyond toxicity, the volume of algal biomass from these blooms creates other problems. When these populations die, their decomposition by bacteria consumes vast amounts of dissolved oxygen.
This oxygen depletion leads to conditions of low oxygen (hypoxia) or even no oxygen (anoxia), creating “dead zones” where most fish, shellfish, and other marine organisms cannot survive. Excess nitrogen also impacts sensitive marine habitats. Coral reefs, which thrive in low-nutrient waters, can be smothered by algal overgrowth fueled by nitrogen pollution. This nutrient enrichment can slow coral calcification, increase their susceptibility to disease and bleaching, and alter their symbiotic relationships. Seagrass beds, other important coastal ecosystems, are vulnerable to degradation from excessive algal growth and reduced light penetration caused by eutrophication.